This invention relates to optical recording media, and in particular, to such media in which means are provided for protecting the surface of an optically recordable layer from scratches, dirt, oxidation or other forms of chemical attack, or the like, which could optically interfere with recording and playback operations.
Various materials, configurations and methods have been earlier proposed to protect optical recording media. For example, in U.S. Pat. No. 4,037,251 (Bricot et al.) an optical recording medium is depicted in which a transparent protective layer is provided on top of a layer in which information, in the form of relief impressions, has already been formed. Not only must such a medium be formed in multiple steps, the last step being the application of the protective layer, but further, such a step must be performed after the medium is recorded upon, such as by a customer to whom recordable record blanks are provided, rather than allowing a protected record blank to be provided to begin with. Also, once the recordable layer is so protected, additional information cannot be readily recorded thereon, as is desirable with direct-read-after-write (DRAW) media, since the protective layer interferes with optical recording processes involving physical or chemical changes, to wit, ablation, gas evolution, bubble formation, etc.
In U.S. Pat. No. 4,308,545 (Lehureau et al.) an optical record disc is depicted which would avoid some of the limitations noted above, since a protective cover is provided as an intergral part of the blank record disc. Such a cover, purportedly made by inexpensively punching and shaping a metal sheet, provides an air gap between the inner surface of the cover and the surface of the light sensitive layer. Since the cover is metal, it is opaque and hence its use is restricted to an optical recording medium having a transparent substrate through which a record and/or playback light beam may pass.
In U.S. Pat. No. 4,331,966 (Moe), an optical record disc is depicted in which glass protective layers and an outer annular ring are combined to enclose a base member having a recessed annulus within which an information storage region is positioned, which top protective layer thus forms a protected space over the information storage region and a circumferential gap is provided to avoid stresses due to differential expansion effects. The assembly depicted is complex and requires numerous precisely machined or punched components, while the incorporated glass members are subject to shock induced breakage.
A simpler protected optical recording medium is depicted in U.S. Pat. No. 4,074,282 (Balas), wherein a disc shaped member having a radiation sensitive layer on one surface is maintained in a spaced relationship from a top protective transparent member via concentric sealing rings, which assembly fails to provide any protection against stress, differential expansion effect and the like.